Presently available methods for recovery of values from lower grade and finely ground ore include froth flotation and hydrocyclone classification. Froth flotation is the attachment of mineral particles and air bubbles in an aqueous media, the transportation of the mineral-bubble aggregates into the froth phase, and the physical separation of the froth from the aqueous mineral slurry. Selective separation of minerals by froth flotation can be obtained by utilizing the hydrophobic or hydrophilic surface properties of the minerals. Hydrophobic minerals attach to bubbles upon collision whereas hydrophilic minerals do not attach to bubbles. Flotation reagents are used to enhance or establish these surface properties. Activator and collector reagents are used to make hydrophobic mineral surfaces and depressant reagents are used to make hydrophilic mineral surfaces. By judicious use of flotation reagents, selective flotation separation of minerals can be accomplished. The conventional flotation cell is basically a stirred-tank with provisions for injecting air as small bubbles. The top of the flotation cell is usually open allowing the mineral laden froth to form and be removed from the cell. Flotation is used almost universally in the minerals industry to beneficiate copper, lead, zinc, iron, phosphate, potash, coal, and many other mineral systems. [See, Ahmed, N. and Jameson, G. J., 1985, "The Effect of Bubble Size on the Rate of Flotation of Fine Particles," International Journal of Mineral Processing, vol. 14, Elsevier Science Publishing Co., pp. 195-215; Jowett, A., "Formation and Distruption of Particle-Bubble Aggregates in Flotation", Fine Particles Processing, Editor, P. Somasundaran, SME-AIME, Volume 1, p. 720, February, 1980; Kelley, E. G. and Spottiswood, D. J., 1982, "Introduction to Mineral Processing", John Wiley and Sons; and Trahar, W. J. and Warren, L. J., 1976, "The Floatability of Very Fine Particles--A Review", International Journal of Mineral Processing, vol. 3, pp. 103-131.]
Hydrocyclones are used universally by the minerals industry for classifying particles by size or density. The swirling flow of the pulp creates centrifugal forces that rapidly accelerate the particles to the peripheral of the hydrocyclone chamber. Coarse particles and/or high density particles quickly move through the fluid and exit through the apex. The fine and/or light particles move slower and are swept into the vortex and removed through the vortex finder. By adjusting the flowrate, hydrocyclone diameter, vortex finder diameter, and apex diameter, a size or density separation can be made.
An "air-sparged hydrocyclone" was developed and patented by J. D. Miller, that combined a hydrocyclone with flotation. [See, U.S. Pat. No. 4,279,743 issued July 21, 1981 to J. D. Miller, "Air-Sparged Hydrocyclone and Method"; Miller, J. D., "The Concept of an Air-Sparged Hydrocyclone", AIME Annual Meeting, Chicago, February 1981; and Miller, J. D., and VanCamp, M. C., "Fine Coal Cleaning with an Air Sparged Hydrocyclone", AIChE National Meeting, paper number 62b, Houston, April 1981.] Like a hydrocyclone, the mineral pulp is injected tangentially, but the air is forced through the porous walls of the hydrocyclone interior. The bubbles move to the hydrocyclone vortex gathering hydrophobic particles and forming a mineral laden froth that exits through the vortex finder. The hydrophilic minerals do not attach to the bubbles and exit through the cyclone apex. Several effective flotation separations have been made with this device.
A major design limitation of the air-sparged hydrocyclone is that bubble size is difficult to control. Bubble size is fundamentally determined by the surface velocity of the pulp which shears the bubble from the pore openings. However, the slowest velocity within the hydrocyclone occurs at the porous interior walls. This low velocity leads to relatively large bubbles (approximately 1 mm diameter size). These bubbles have a low probability for collision with the fine mineral particles (less than 10 .mu.m size). To increase the collision probability for fine size mineral particles, either more bubbles, smaller bubbles, or higher shear velocities are needed. Unfortunately, more air leads to bigger bubbles. At increased pulp flowrates, the shear velocity increases making smaller bubbles, but the concentration of bubbles declines because the flowrate has increased. To a large extent, the smallest bubble size is fixed by the basic design of the air-sparged hydrocyclone and cannot be lowered without decreasing the bubble-particle collision probability.
Those concerned with these and other problems recognize the need for an improved mineral separation apparatus and method.